What is the FUSE Project?

For hundreds of years astronomers observed the Universe using only the visible light our eyes can see. However, visible light is a tiny portion of a much broader range of light energy known as the electromagnetic spectrum, which includes everything from energetic X-rays and gamma rays to infrared radiation and radio waves. Much of this "invisible" light gets blocked by the Earth's atmosphere, but in the last forty years astronomers have been using telescopes above the atmosphere to obtain entirely different perspectives on the Universe. A new perspective, one that has only been glimpsed a few times before, is being provided by a telescope known as the Far Ultraviolet Spectroscopic Explorer, or FUSE. Funded by NASA as part of its Origins program, FUSE was launched into orbit aboard a Delta II rocket on June 24, 1999 for at least three years of operations.

FUSE was developed and is being operated for NASA by the Johns Hopkins University. FUSE was developed in collaboration with the space agencies of Canada and France, who shared in the observing time over the first three years. This is the first time that a mission of this scope has been developed and operated entirely by a university.

FUSE was designed for a very specialized and unique task that is complementary to other NASA missions. FUSE looks at light in the far ultraviolet portion of the electromagnetic spectrum (approximately 90 to 120 nanometers), which is unobservable with other telescopes. FUSE observes these wavelengths with much greater sensitivity and resolving power than previous instruments used to study light in this wavelength range.

The FUSE satellite consists of two primary sections, the spacecraft and the science instrument. The spacecraft contains all of the elements necessary for powering and pointing the satellite: the attitude control system, the solar panels, communications electronics, and antennas. The science instrument collects the light of distant objects and contains the equipment necessary to disperse and record the light: the telescope mirrors, the spectrograph (and its electronic detectors), and an electronic guide camera called the Fine Error Sensor (or FES). The spacecraft and the science instrument each have their own computers, which together coordinate the activities of the satellite.

Astronomers will view the Universe in a whole new light using the unique data obtained with FUSE. In particular, they seek answers to long-standing questions such as: "What were the conditions like in the first few minutes after the Big Bang?" ,"How are the chemical elements dispersed throughout galaxies, and how does this affect the way galaxies evolve?", and "What are the properties of the interstellar gas clouds out of which stars and solar systems form?" All of these questions, and many others, can be addressed by observing the far ultraviolet light from stars, interstellar gas, and distant galaxies with FUSE.

The scientific approach of the FUSE mission is special because a science team has been charged by NASA with providing answers, or at least partial answers, to intriguing questions like those posed above. Toward this end, the FUSE science team will undertake a comprehensive study of the cosmic abundance of deuterium, a rare form of "heavy hydrogen" formed only in the Big Bang. The team will also study the hot gas content of our galaxy, the Milky Way, and its nearest neighbor galaxies, the Magellanic Clouds. To conduct these large studies, the FUSE science team will observe hundreds of astronomical objects, using about half of the observing time during the three-year mission. The remaining observing time is devoted to a Guest Investigator program where NASA selects scientific investigations proposed by astronomers world-wide.

What Will FUSE Explore?

Deuterium and the Big Bang

In the infancy of the Universe, the extreme conditions present everywhere gave rise to the creation of simple chemical elements out of which all matter was made. The simplest element, hydrogen, consists of a positively charged nucleus containing a single proton orbited by a negatively charged particle known as an electron. In some instances, these hydrogen atoms also have a second particle called a neutron in the nucleus accompanying the proton; this type of hydrogen is called deuterium. More complicated elements consist of atoms having larger numbers of protons and neutrons in their nuclei surrounded by correspondingly higher numbers of electrons.

When atomic nuclei formed in the early Universe, the conditions were so severe that electrons were unbound to the nuclei and moved about freely. Gas with this property is known as plasma. In this plasma, some of the hydrogen was converted to deuterium, and some of the deuterium was converted to helium. The relative amounts of each element produced by this nuclear fusion of protons and neutrons were very sensitive to the temperature, density, and number of the particles in the plasma at that early time. As the Universe expanded, the plasma cooled, the creation of elements ceased, and the free electrons and nuclei combined to form complete atoms.

It is the sensitivity of the nuclear reactions in the primordial plasma to the initial conditions in the Universe that makes astronomers interested in studying the simple elements today. By measuring the relative amounts of each element, it is possible to infer the conditions present at a time before complete atoms existed! In particular, knowing the ratio of deuterium atoms to hydrogen atoms left over from the Big Bang would allow astronomers to place a strong constraint on how much observable matter there is in the Universe.

Alas, Nature does not reveal secrets such as these so easily ­ the abundances of some elements have changed over time. The interior cores of stars are hot enough (tens of billions of degrees) to mimic those conditions in the first few minutes of the Universe and convert deuterium into helium by the addition of another proton to the deuterium nucleus. Unlike the early Universe, however, the nuclear reactions in stars are sustained over very long periods of time, which means that fragile light elements like deuterium can be readily converted into much heavier elements. For this reason, astronomers believe that the total amount of deuterium in the Universe is decreasing as matter gets cycled through stars, but they do not know how fast it is decreasing or how much deuterium has already been destroyed.

This is where FUSE enters the quest to understand our cosmic origins. Astronomers will use FUSE to search for deuterium in the interstellar medium near the Sun, in gas clouds in the far reaches of the Milky Way, and in distant intergalactic clouds between galaxies. By measuring the amount of deuterium relative to both hydrogen and the heavier elements produced by stars, they will be able to estimate how much deuterium has been destroyed since the Big Bang. This, in turn, will allow them to understand how galaxies evolve and to discover what the Universe was like when it was only a few minutes old.

The Chemical Evolution of Galaxies

Galaxies like our own are massive collections of stars, gas, and dust. Matter and energy are exchanged between these various components in a grand cycle that changes the chemical and physical properties of galaxies. Stars form from the interstellar material, synthesize chemical elements in their interiors, and return their products to the interstellar gas during their lives and in their death throes. All naturally occurring elements heavier than lithium are produced by these cycles. The carbon atoms that form the basis of life, the oxygen we breathe, and the silicon in the sand on our beaches are all formed deep inside some previous generations of stars. The calcium in our teeth, the copper in our currency, and the iron in the steel frames of our cars are formed in massive stellar explosions called supernovae that occur as stars exhaust their nuclear fuel, collapse under their immense weight, explode, and reseed the interstellar gas for a new generation of stars.


The beautiful Horsehead Nebula in Orion dramatically demonstrates the presence of gas and dust in the vast regions of space between the stars. (Image © Anglo-Australian Observatory.)

Understanding how stars and the interstellar medium interact with each other is a major concern of astronomers. The energy produced by stars is shared with the interstellar medium as stellar winds sweep up gas and dust, and stellar explosions vacate large cavities and create "bubbles" filled with very tenuous, hot gas. This stellar activity can trigger interstellar gas clouds to collapse and form new stars and solar systems, or it can disrupt the very same processes and prevent them from occurring.

One of the major predictions of theories for these interactions is that some portion of the interstellar medium should be heated to very high temperatures by all this activity. In the hot gas, atoms are ionized ­ that is, the electrons that normally surround the atomic nuclei are stripped off the atoms. As the gas cools, some of the electrons reattach to the positively charged ions. One of the most important ions that astronomers can observe is oxygen that has had five of its eight electrons removed; this form of oxygen is called O VI ("oxygen six"). It is a very good indicator of gas that has been heated to temperatures of one million degrees or more and is cooling as the recombining electrons and ions emit or absorb light.


The graceful arcs of the Vela supernova remnant are seen against the rich star field of the Milky Way. These gaseous filaments arise where the 10,000 year old supernova blast wave has swept up and heated the tenuous interstellar gas. (Image © Anglo-Australian Observatory.)

FUSE is designed to make very sensitive measurements of O VI in the interstellar medium and the remnants of supernova explosions. One of the primary scientific objectives of the FUSE mission is to determine whether a large halo of hot gas surrounds our galaxy. By studying the distributions of O VI and many other atoms and ions, astronomers will be able to determine the composition of the interstellar gas, how well it is mixed, and which processes are effective in heating the gas. All of this information can be used to help us understand how galaxies evolve and form new generations of stars and planets.

More on FUSE Science:
General Description or More Technical Description

How Does FUSE Work?

To accomplish its task, FUSE incorporates a number of unique design features. For instance, instead of a single mirror FUSE uses four mirror segments to reflect the light to focus. Two mirror segments are coated with a material (silicon carbide) that has superior reflectivity at the shortest ultraviolet wavelengths, and two mirror segments are coated with a different material (aluminum and lithium fluoride) that reflects better at longer wavelengths. This optimizes performance over the entire spectral range. FUSE also uses two sophisticated electronic detectors to "see" the incoming ultraviolet light and record it digitally for downlink to the ground.

The ultraviolet light seen by FUSE is dispersed (or broken up) into a spectrum by four special optical components called gratings (one for the light from each of the four mirror segments). The FUSE gratings are quite large, and have been etched with a very large number of fine, parallel grooves. The grooves disperse the light into a spectrum, and the large number of grooves packed closely together provides the high resolving power (ability to see details) that allows FUSE to do its job. The FUSE gratings are curved instead of flat, which made their manufacture very complex.

The Fine Error Sensor (or FES) is basically the "eyes" of the satellite. The FES works in visible light, and images a region about a third of a degree in size in the direction that the telescope is pointing. (For comparison, the moon is about half a degree across.) The FES can see stars down to about 14th magnitude, which is about 5,000 to 10,000 times fainter than you can see on a typical clear night! The FES produces the only "pictures" that we will get from FUSE; the real job of FUSE is to observe the spectrum of astronomical objects in far-ultraviolet light invisible to ground-based telescopes. Analysis of these spectra provides a wealth of information about the object being observed and any gas or dust along the line-of-sight that may absorb some of the light along the way.

More on FUSE itself:
General Hardware Specs, or FUSE Animated Light Path or Technical Description

FUSE Operations

FUSE is controlled through a primary ground station antenna located at the University of Puerto Rico, Mayaguez. The satellite's circular 775 kilometer (480 mile) orbit, which takes about 100 minutes for a single revolution, brings it over the ground station for less than 10 minutes at a time (on average) for about six orbits in a row, followed by roughly eight orbits without contact. Hence, the satellite must operate on its own most of the time, moving from target to target, identifying star fields, centering objects in the spectrograph apertures, and performing the observations. The scientific data, which are stored in digital form, are radioed to the ground during contacts with the ground station.

All of the instructions the satellite needs to perform its tasks are pre-planned and uplinked to the onboard computer during contacts with the ground station. Preparation of these instructions occurs in the Satellite Control Center located in the Bloomberg Center for Physics and Astronomy at The Johns Hopkins University. Potential observations are scheduled based on predicted viewing intervals, spacecraft positioning constraints, and the needs of each science program. These schedules, or timelines of activities, are then turned into detailed instrument instructions and uplinked to the satellite by a team of engineers. The observations normally take place without direct interaction by ground controllers.

More on FUSE operations:
General Description or Satellite Control Center or Ground Station or Data Handling.

FUSE Primary and Extended Missions

The first 3-1/2 years of FUSE operations were dubbed the Primary Mission. During this period, the observing time on FUSE was shared roughly 50-50 between the FUSE science team and a host of Guest Investigators, astronomers from around the world selected by NASA to participate in the FUSE program. As of April 1, 2003, the FUSE project is in an extended phase of operations. With continued funding from NASA, the FUSE satellite continues to be operated as an observatory for the astronomical community, with 100% of on-orbit observing time selected by NASA peer review. Some 29 million seconds of science data were obtained during the Primary Mission phase.

The Extended Mission period puts forth a number of challenges, especially for satellite operations. Many procedures have been automated to the extent possible, allowing the project to cut back on staffing and minimize operations costs. As one example, the Satellite Control Center was staffed around the clock during the Prime Mission. but has now transitioned to a 16 hour per day, Monday through Friday staffing profile in the Extended Mission. Less redundancy and less access to ongoing engineering support is a standard situation for missions in their extended phase, where a higher level of risk is allowed.

FUSE's Principal Investigator

Dr. Warren Moos is Professor of Physics and Astronomy at the Johns Hopkins University. He is a specialist in space optics and ultraviolet instrumentation. In addition to being the Principal Investigator of the FUSE mission, he is a co-investigator for the Space Telescope Imaging Spectrograph installed on the Hubble Space Telescope in 1997 and a co-investigator for the Hopkins Ultraviolet Telescope flown on Space Shuttle flights in 1990 and 1995. Dr. Moos was also a co-investigator for the Voyager UVS and the Apollo 17 UVS experiments and has been an extensive user of space telescopes. Dr. Moos has served previously as Director of the Center for Astrophysical Sciences and as Chair of the Department of Physics and Astronomy at the Johns Hopkins University.

FUSE Partners

FUSE is a joint project of the National Aeronautics and Space Administration and the Johns Hopkins University in collaboration with:

Centre National d'Etudes Spatiales (France), the Canadian Space Agency, the University of Colorado, and the University of California, Berkeley.


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